THE EFFECT OF COVER CROP AND CROP ROTATION ON SOIL WATER STORAGE AND ON SORGHUM YIELD

Crop rotation and cover crop can be important means for enhancing crop yield in rainfed areas such as the lower Coastal Bend Region of Texas, USA. A trial was conducted in 1995 as part of a long-term cropping experiment (7 years) to investigate the effect of oat (Avena sativa L.) cover and rotation on soil water storage and yield of sorghum (Sorghum bicolor L.). The trial design was a RCB in a split-plot arrangement with four replicates. Rotation sequences were the main plots and oat cover crop the subplots. Cover crop reduced sorghum grain yield. This effect was attributed to a reduced concentration of available soil N and less soil water storage under this treatment. By delaying cover termination, the residue with a high C/N acted as an N sink through competition and/or immobilization instead of an N source to sorghum plants. Crop rotation had a significantly positive effect on sorghum yield and this effect was attributed to a significantly larger amount of N concentration under these rotation sequences.


INTRODUCTION
Available soil water and soil N are two factors of major importance in crop production.Cropping prac-tices, such as use of cover crop in rotation systems, used to improve soil moisture storage and accumulation of soil nitrate-N from decomposing organic matter would stabilize crop yields in rainfed areas such as the Lower Coastal Bend Region of Texas.
Cover crop can maintain or increase crop yield by reducing soil erosion, increasing soil organic matter, improving soil physical properties, reducing water runoff, reducing chemical inputs and maintaining or improving water quality (Holderbaum et al., 1990;Meissinger et al., 1991).Response to dry-matter accumalation and early season soil water storage by cover crop depend basically on three factors: weather, soil-holding capacity, and cover crop management (Wagger, 1987;Wagger & Mengel, 1988).Under humid conditions, soil water depletion by cover crop is not a critical factor particularly on soil with high water-holding capacity.On soil with low water-holding capacity, dry weather conditions and erratic distribution of summer rainfall often fails to recharge profile soil moisture during the growing season.This condition would be more aggravated by the presence of cover crop prior to planting the summer crop.To minimize the effect of soil-water depletion by cover crops, early herbicide desiccation, animal grazing, or mechanical harvesting could be used (Campbell et al., 1984).The effectiveness of a cover crop in maintaining or improving soil water storage also depends on other factors such as: cover crop species, method of planting, stage of growth, length and growth period, method of extermination, leaving the crop as a mulch or green manure (Moschler et al., 1967;Nelson et al., 1977;Campbell et al., 1984).
The objective of this research was to investigate the effect of cover crop and crop rotation on soil water storage and to grain yield of sorghum.

MATERIAL AND METHODS
This study was part of a long-term rotation sequence initiated in 1988 at the Texas A&M Agricultural Research and Extension Center, at Corpus Christi, USA, to evaluate cropping systems suitable for the lower Coastal Bend Region of Texas.The experimental site included three soil types: Victoria clay soil series (fine montmorillonitic, hyperthermic, Type Pelluster), Clareville complex (Fine, montmorillonitic, hyperthermic Pachic Argiustolls), and Orelia fine sandy loam (Fine loamy, mixed hyperthermic Typic Ochraqualfs).
The experimental design was a randomized complete block in a split plot arrangement with four replicates.Crop rotations consisted of continuous sorghum, sorghum/cotton, sorghum/soybean, continuous cotton, cotton/sorghum, cotton/soybeans, soybean/sorghum, and soybean/cotton and were the main plots; cover crop (cover and no cover) was the subtreatment.Cover crop consisted of seeding feed grade oats in late September 1994 at a rate of approximately 170 kg/ha and receiving glyphosate ( ( N -p h o s p h o n o m e t h y ) g l y c i n e ) a n d 2 , 4 D ((2,4 -diclorophenoxy) acetic acid) at a rate of 2,4 L/ha on January 13, 1995.The desiccated cover was allowed to remain in the field to provide a surface mulch until spring planting preparation; then, it was incorporated to the soil.Plant tissue samples were taken from the cover crop prior to extermination for dry matter yield and tissue analysis.
Data presented in this paper were collected in the 1995 growing season using only sorghum rotation sequences.Each plot was 13.7 m wide and 61 m long.
Sorghum rotations received preplant broadcast applications of 34 kg/ha of N (32-0-0).A starter fertilizer, 8-24-2 rate of 30 L/ha was also applied in 10 cm band over the seed furrow at planting.Sorghum cultivar DEKALB 56 was sown in 96 cm rows on March 26, 1995, at a population of 110,000 plants/ha.Weeds were controlled with a pre-emergence application of atrazine at a recommended rate.Two cultivations on April 24 and May 17 were needed to control remanecent weeds.
Soil water contents were monitored throughout the season using a neutron probe (Campbell Pacific Model 503,CPN Crop.,Martinex,CA).Access tubes were installed in two locations (North and South) of each subplot at a sorghum/cotton rotation of four replicates after plant emergence.The probe was calibrated at each site by obtaining 15 cm increment soil samples and determining the soil water content.The gravimetric data were converted to a volumetric basis by multiplying them by the soil bulk density.Probe readings were taken twice a month during the growing season.Readings were taken from surface to a depth of 90 cm.Neutron probe readings were converted to volumetric soil water contents by use of a regression equation to determine the amount of water in the soil profile.Volumetric soil values for the experimental area ranged from approximately 42% at saturation to 34% at field capacity to 18% at permanent wilting point (Lawlor et al., 1992).Plant extractable water (PEW) was determined as the difference between the volumetric soil moisture (VSM) measured between dates when little or no rainfall occurred.This is a modified laboratory and field method used by Ratliff et al. (1983) and Ritchie (1981), cited by Lawlor et al. (1992) to determine potential extractable water.Soil water depletion (WD) was determined as the difference between total water content on April 28 and July 28.Evapotranspiration (ET) was calculated as the sum of soil water depletion (WD) in the whole profile and precipitation received during the same experimental period, i.e., ET=WD + rainfall.Water runoff was assumed negligible because the slope was <2%.Water use efficiency (WUE) was calculated as gain yield (Y) divided by ET, or WUE=Y/ET (Tanner & Sinclair, 1983).Data were analyzed by the standard analysis of variance procedures.

RESULTS AND DISCUSSION
Oat dry matter and nitrogen content Dry matter yield (DM) and N concentration constitute an estimation of the N content of the above ground portion of a cover crop.These values represent N uptake from residual soil inorganic N, mineralized N, and biologically fixed N in legume cover crop.Mean values of oat DM and N content harvested in January of 1995 are presented in Table 1.Oat yield ranged from 1,449 kg/ha to 1,842 kg/ha.Oat yield was higher under rotation when compared with continuous sorghum.Sorghum/cotton was the crop sequence that improved most cover crop dry matter yield.For N content, the highest mean value was registered under sorghum/cotton rotation (23.6 g/kg) which differed significantly (P<0.05)only from sorghum/sorghum sequence.This result was somehow expected because of the higher nutrient demand by continuous sorghum sequence.

Soil organic matter and soil nitrogen
Results of organic matter (OM) and total N determinations are presented in Table 2. ANOVA indicates that only the main effect of rotation and depth on OM were significant (P<0.05 and P<0.01, respectively).Higher OM means were registered under rotations which did not significantly differ from each other but differed from continuous sorghum.Analysis of variance for this variable indicates a significant rotation x cover crop interaction (P<0.05).Soil N content was generally higher under sorghum/cotton and sorghum/soybean.
Within cover crop regime, no significant difference was observed between sequence treatments.Within no cover regime, the largest N concentration was registered on sorghum/cotton, which did not differ from sorghum/soybean, but differed from continuous sorghum.Cover crop significantly reduced soil N concentration.

Soil water content and crop water use
Average air temperature and cumulative precipitation for a seven-day period during the growing season are presented in Table 3. Rainfall from March to August 1995 was 96% of the 15-year average precipitation of 369 mm.Average temperatures were adequate for optimum growth and development of sorghum.However, seasonal variations in the amount and distribution of rain occurred throughout the season.Severe soil moisture deficit was observed at the critical stage of plant development, i.e, from planting-to-bloom period (1 -60 days after planting).

TABLE 1 .
Dry matter yield and nitrogen content of oat leaves at harvest.Corpus Christi, TX, 1995.

TABLE 2 .
Effect of crop rotation and cover crop on soil organic matter and nitrogen.Corpus Christi, TX, 1995 1 .

TABLE 3 .
Weekly mean temperature and cumulative precipitation during the growing season.Corpus Christi, TX, 1995.

TABLE 6 .
Mean profile soil water content on sorghum/cotton rotation as influenced by cover crop.Corpus Christi, TX, 1995 1 .

TABLE 7 .
Mean profile plant extractable water influenced by cover crop on sorghum/cotton rotation.Corpus Christi, TX, 1995 1 .Means followed with the same letter in a column are not significantly different.2Calculated as initial soil water content minus final soil water content. 3Calculated as WD plus precipitation. 4 Calculated at Y/ET. 1

TABLE 9 .
Sorghum grain yield as affected by rotation and cover crop.Corpus Chisti, TX, 1995 1 .